A wireless communications system refers generally to any telecommunications system which enables wireless communication between the users and the network. In mobile communications systems, users are capable of moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN).
At present, third generation mobile systems, such as the Universal Mobile Communication System (UMTS) and the Future Public Land Mobile Telecommunication System (FPLMTS), later renamed IMT-2000 (International Mobile Telecommunication 2000), are being developed. The UMTS is being standardized at ETSI (European Telecommunication Standards Institute), whereas ITU (International Telecommunication Union) is defining the IMT-2000 system. The radio interface of 3G mobile systems is likely to be based on a wideband CDMA (code division multiple access), and therefore third generation systems are often referred to as Wideband CDMA systems (WCDMA). These future systems are basically very alike.
FIG. 1 illustrates a telecommunications network in which the invention is applicable. The telecommunications network could be e.g. a third generation cellular mobile network, such as the UMTS. The telecommunications network comprises a first end node, such as a mobile station MS (referred to as a User Equipment UE in the UMTS terminology), and a second end node, such as a Radio Network Controller RNC1, RNC2. The network may also comprise several middle nodes, such as Base Stations BS1 to BS4. Information to be sent between the end nodes is formatted as frames (e.g. RLC PDUS). There may be several (two or more) parallel connections between the end nodes via several middle nodes, enabling macrodiversity, diversity combining or soft handover.
A copending Finnish patent application 982399, filed on Nov. 5, 1998, by the same Applicant, discloses a new synchronization mechanism, referred to as a Connection Frame Number (CFN) scheme, for avoiding synchronization problems especially in macrodiversity implementations. According to the current vision of UMTS, some traffic overhead is eliminated by not transmitting frame numbers with the frames (i.e. on a traffic channel) over the radio interface Uu. Instead, in the BS to UE direction, the frame numbers are broadcast to all mobile stations simultaneously, and in the BS to RNC direction, the base stations add the frame numbers in a modulo-p sequence where the currently proposed value for p is 72. In other words, the frame numbers are repeated cyclically: 0, 1, . . . , 71, 0, 1, etc. The base stations are not synchronized with each other. Therefore, the frame numbers are relative and, indeed as such, they are meaningless without at least implicit information about the timing reference on which the frame numbers are based. Because the frame number is not transmitted over the radio interface, and because the base stations use different timing references, a frame N sent by the mobile station to the BS1 and BS2 may be forwarded by the BS1 as frame N′ to the RNC, whereas BS2 would send the same frame as frame N″ to the RNC. As a consequence, diversity combining at the RNC fails.
In the new mechanism suggested by FI982399, and also accepted in present UMTS scenarios, a connection-specific timing reference (called CFN), which is common to all nodes involved in the connection, is established so as to enable any middle node (BS) to determine and compensate for an offset between its local timing reference and the CFN. The end nodes (RNC and UE) agree the CFN at the beginning of the connection by means of outband signaling. The CFN is tied to the timing of a broadcast control channel (BCCH) in the cell, i.e. it is incremented every 10 ms. A number of RLC PDUs are incorporated into a larger data unit, a Transport Block Set (TBS), that is transmitted in one interleaving period (a multiple of 10 msec radio frames). Thus, the CFN is ‘transmitted’ through the transport channel between the UE and the RNC, i.e. the CFN is incremented locally both at the UE and the RNC, associated with each TBS. In case of interleaving periods longer than 10 msec (i.e. the TBS extends over two or more radio frames), the CFN refers to the first radio frame utilized for the transmission of the interleaving block.
To meet the requirements for the length of the frame number used for ciphering, an extension of the CFN, a Hyper Frame Number HFN, is provided. The length of the HFN is at least 25 bits so that the total length of HFN+CFN is at least 32 bits. The HFN is initialized to a common value in the UE and SRNC, and then it is incremented at every completed CFN cycle, i.e. every 72 msec. As a result, a configuration as shown in FIG. 2 is provided for frame numbering and synchronization.
The above Finnish patent application FI982399 further proposes, as is accepted also in the present UMTS scenarios, to use the UEFN (i.e. CFN+HFN) as a cipher key, since the length of CFN+HFN is sufficient for reliable ciphering. Other inputs to the ciphering algorithm may be a ciphering key Kc, a Bearer ID and a Direction, as illustrated in FIG. 3. Kc:s are calculated in the UE and SRNC during the authentication procedure. The direction parameter defines in which direction the data is sent (Uplink/Downlink). The bearer ID (RAB ID) is a radio access bearer or a signaling link specific parameter, unique within one RRC connection. It is used as input parameter for ciphering to ensure that the same ciphering mask is not applied to two bearers that have the same Kc and are transmitted with the same UEFN (in case of L1 or MAC multiplexing). The ciphering algorithm generates a ciphering mask. It is likely that radio interface ciphering in the UMTS will be a MAC functionality which allows the encryption/decryption of MAC SDUs (RLC PDUs) on basis of XOR combining with the ciphering mask. The main benefit of this ciphering solution is the use of one and the same mechanism for all channel types and bearer types.
In the initialization the UE (acting as a master) sets its own reference, UEFN, for frame numbering. The HFN is initialized before ciphering is started, e.g. during setup of the RRC connection or during the ciphering mode setting procedure, and is maintained (run) all the time the UE is in RRC connected mode. The HFN is preferably initialized to a value that is hard to predict by a fraudster. The reason why the HFN should not be initialized to a fixed value (e.g. 0) is to prevent the reuse of the same ciphering masks within too short a period of time in case the Kc is not changed.
The basic unit for ciphering is one RLC PDU, i.e. (in uplink direction) the UE ciphers each RLC PDU with the respective UEFN in the MAC layer prior to sending the frame to the RNC. The MAC layer in the RNC is aware of the UEFN (in accordance with the principles of the CFN concept) used by the UE, and is able to decipher the received frame. As a consequence, the ciphering mask is changed as the CFN or HFN is incremented.
An improvement of CFN-based ciphering is described in a copending Finnish patent application 990499, filed on Mar. 8, 1999, by the same Applicant.
Problems may arise, when “CFN ciphering” is used with some of the Automatic Repeat Request (ARQ) techniques. Generally, the ARQ is a transmission method in which error correction is based on retransmission. A very basic ARQ scheme includes only error detecting and retransmission capabilities. If a packet is found to have errors after decoding, this packet is discarded and retransmission is requested. The source then retransmits an exact copy of that packet. A Hybrid ARQ is a transmission scheme which combines error detection/correction (such as Forward Error Correction, FEC) and retransmission of the erroneous frame. Three types of hybrid ARQ have been defined:
Type I: Type I hybrid ARQ is an Adaptive Coding Rate (ACR) method. The main idea behind the ACR ARQ methods is to vary the coding rate for error correction according to system constraints such as the signal-to-noise ratio in a given environment. With ACR ARQ, whenever a data RLC-PDU is received with an uncorrectable error pattern, the RLC-PDU is discarded and a request for retransmission is sent back through a return channel to the transmitter. The transmitter then resends an exact copy of the original RLC-PDU.
Type II: Type II hybrid ARQ belongs to the Adaptive Incremental Redundancy (AIR) ARQ schemes. In AIR ARQ schemes, a RLC-PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. The advantages of Type II hybrid ARQ, as compared with Type I hybrid ARQ, are that if the interference distribution across a radio cell is such that a significant fraction of RLC-PDUs will be received correctly even with a low initial FEC code rate, then a higher throughput can be achieved. Further, since repeat transmissions can be soft-combined, there is an increase in the probability of correctly decoding the RLC-PDU.
Type III: The retransmitted packed may be combined with the previous versions, if available, but each version contains all the information necessary for correct reception of the data. It does not offer obvious throughput gains when compared with Type I hybrid ARQ, but the combining of consecutive transmissions offers a better decoding probability than repeat transmissions with Type I hybrid ARQ and may be a significant benefit.
Type I hybrid ARQ with soft combining: With Type I hybrid ARQ, it is also possible to store the erroneous packet in the receiver and combine it with the retransmitted packet. This can be considered as incremental redundancy in the form of a repetition code, and therefore this can be considered as a special case of Type II/III hybrid ARQ.
All the hybrid ARQ schemes using soft combining of different transmissions, i.e., Type II and Type III as well as Type I hybrid ARQ with soft combining, require that information about which packets to combine has to be signaled outband in order to be able to associate and soft-combine different versions of the packet. This information can be the PDU number of the packet, for example. The receiver will combine the versions of the packet that are indicated by the outband signaling, e.g. to have the same PDU number. ‘Outband’ means that this information is protected with a separate error detection code (e.g. CRC) and outband information is usually assumed to be more reliable than inband data. A separate, more reliable channel can be used (with more power or a more powerful error correction scheme) or a better protected header with its own CRC. It is also possible to send the data ‘inband’ (using the same channel as the data does) but with better protection. The problem is that the above described CFN-based ciphering method does not work together with hybrid Type II/III ARQ. Since Type II/III hybrid ARQ requires that retransmitted data is exactly identical to the first transmission attempt (to enable soft combining), the basic problem with CFN-based ciphering is how the transmitted knows which CFN should be used in retransmissions and how the receiver knows which CFN should be used for deciphering if retransmissions are needed before data is received correctly. With the ‘normal’ CFN-based ciphering method, the retransmitted RLC PDU uses a ciphering mask different from the initial transmission, which makes the soft combining of the two PDU versions on the receiver side (on L1, below ciphering) impossible. On the other hand, if the ciphered PDU is stored on the L1 layer for retransmission (not reciphered when retransmitted), then soft combining on the receiver side is possible, but the problem is that deciphering is not possible, since the receiver does not know the CFN of the originally transmitted PDU. As a consequence, CFN-based ciphering cannot be used for all types of services in the communication system, but different ciphering methods are needed for bearer services utilizing Type II/III hybrid ARQ.
Similar problems can be encountered in any communications system using an ARQ with soft combining, even without CFN-based encryption.